The electronics industry requires circuit boards which are substantially free of ionic and organic flux residues since such contribute to failure of the circuit board in use. Therefore, stringent methods are employed to clean the boards of both ionic and organic residues. Numerous solvents and mixtures of solvents have been tried and discarded. The most widely used commercial solvent is 1,1,2-Trifluoro-1,2,2-trichloroethane (Fluorocarbon 113) in admixture with 10.67 volume percent methanol and 0.33 volume percent nitromethane. This solvent effectively cleans from the soldered circuit board the rosin flux soldering aids. The cleaning effectiveness is measured by standard procedures in the industry, one of which is set by the U.S. military which is a specification for "Printed Wiring Assemblies" MIL-P-28809. This test consists of spraying or immersing the cleaned board in a freshly prepared aqueous isopropyl alcohol solution for a specified period of time after which the resistivity of the solution is measured in ohm-cm. The effectiveness of a flux removal blend is a function of the cleaning time, flux composition and the type of cleaning operation. All these being equal, the more effective blends will give a higher specific resistance value when tested according to the above test or similar standard test.
The above mentioned fluorocarbon blend has been shown by industrial experience and by means of the above test to be an effective flux removal solvent. Generally, chlorinated hydrocarbons alone or in combination with alkanols below the flash point level give poorer results, particularly with respect to removal of ionic components of the flux. It is important that the blends used by the industry have no flash point for the obvious safety reason.
It is also known that chlorinated hydrocarbons, especially 1,1,1-trichloroethane (methylchloroform), will remove the nonionic components of the rosin flux solder aids better than the aforementioned fluorocarbon blend.
Patents disclosing flux removing compositions are U.S. Pat. Nos. 3,932,297 and 4,023,984, claiming methylchloroform with n-propyl alcohol and isopropyl alcohol, respectively; U.S. Pat. No. 3,733,218 discloses flux removing blends of perchloroethylene and trichloroethylene, each with different alcohols which form azeotropes. An azeotropic composition of a fluorocarbon and n-butyl alcohol is disclosed in U.S. Pat. No. 3,671,446 as useful in cleaning circuit boards.
It, therefore, would be advantageous to have a chlorinated solvent composition which will effectively remove both ionic and nonionic flux residues and has no flash point. The present invention provides such a composition which comprises a stabilized methylchloroform in combination with a mixture of methylbutynol and 2-butanol.
There are numerous patents which disclose the use of various alcohols as stabilizers for methylchloroform. Among these are U.S. Pat. Nos. 3,049,571; 3,192,273; 3,326,988; 3,360,575; 3,499,047; 3,565,811; 3,746,648; 3,974,230; 4,018,837; 4,115,461 and 4,324,928. Alcohols included in these patents are 2-butanol, 2-methylbutan-2-ol, 2-methylpropan-2-ol, butynol and 3-methyl-1-butyn-3-ol in combination with various other known stabilizers, e.g. nitromethane and/or nitroethane, butylene oxide, dioxane, and the like, for methylchloroform.